This invention relates generally to methods and compositions for hydraulic fracturing of subterranean formations. More particularly, this invention relates to a fracturing method which includes the steps of injecting fluids into adjacent formation zones wherein the fluids are preferably capable of reacting with one another at the interface between the two zones to form a fracture-arresting product, preferably a precipitate.
Hydraulic fracturing is a well-known operation used to stimulate oil production. Generally, hydraulic fracturing involves injecting a fracturing fluid into a subterranean oil-bearing formation at an elevated pressure to increase the permeability of the formation. Typically, the fluid is introduced into the formation through a conduit, such as the drill pipe, tubing, or casing. The fluid moves down and outward into the oil-bearing formation from the well bore at a sufficiently high rate and pressure to create fractures and cracks. The minimum downhole pressure required to induce fractures in the formation is often referred to as the "fracture gradient," and is sometimes expressed in terms of p.s.i. per foot of depth from the surface.
The fluids typically used in hydraulic fracturing may comprise any number of materials, including but not limited to water, oil, alcohol, dilute hydrochloric acid, liquified petroleum gas, or foam. In addition to these fluids, solid particles known as propping agents or "proppants" may also be introduced to the formation through the well bore. These proppants, such as sand grains, pellets, or glass beads, fill fractures created during the high pressure stages of the fracturing operation and leave channels for oil to flow through when the pressure is released at the surface.
Subterranean formations typically comprise a number of levels or zones which run substantially horizontally and are layered vertically. Each zone, composed of materials such as rocks, sands, and limestones, has a permeability, porosity, and other properties which is often different from an adjacent zone. One of these properties, of particular interest to the present discussion, is stress. The term "stress," as used herein, refers to tectonic F-forces which occur naturally in subterranean formations and which result from pressures exerted on the zone from different directions. It is recognized that fractures propagate proportionally and in a direction normal to the "minimum" or "least" stress occurring in the formation. Accordingly, the term "stress" as used herein means "minimum stress" unless otherwise provided. Generally, because this minimum stress usually lies in the horizontal direction, fractures tend to propagate vertically. The terms "low stress" and "high stress" as used herein are intended to be relative to one another. Thus, for example, any zone adjacent to a zone of interest having a lower minimum stress than that of the zone of interest is a "low stress zone," while the zone of interest is the "high stress zone."
Some of the problems with hydraulic fracturing include unintended crack propagation and uncontrolled fracture height growth. Often, for example, hydraulic fractures induced in an oilbearing formation eventually "propagate" by spreading into adjacent zones or bounding formations. This propagation has been particularly troublesome in situations where the oil-bearing zone of interest or "pay zone" has an equal or higher minimum stress than the minimum stress of an adjacent zone. It has been discovered that, in such situations, fractures induced in the pay zone tend to propagate toward the adjacent zone. This tendency of fractures to propagate toward a lower stress zone is discussed in an article by W. El Rabaa, entitled "Hydraulic Fracture Propagation in the Presence of Stress Variation," SPE 16898, 205-18, 62nd Annual Technical Conference and Exhibition of the Society of Petroleum Engineers (Dallas, Texas, Sept. 27-30, 1987). Such fracture propagation may have serious consequences. For example, proppant materials injected into a zone of interest may leak into the adjacent zone. Consequently, fractures induced in the zone of interest, lacking sufficient proppant materials, may "heal" after the pressure is released, possibly requiring another fracturing operation. A further problem is that fractures which have spread into the adjacent zone may remain open after the fracturing operation so that petroleum may leak from the zone of interest into the adjacent zone, resulting in inefficient recovery of petroleum.
The present invention accordingly provides an improved method of hydraulic fracturing which addresses the aforementioned problems. Preferably, this invention controls and/or arrests fracture growth, and in general improves the effectiveness of a fracturing operation, resulting in an improved fracture pattern having reduced propagation from the pay zone into an adjacent zone.